1. Field of the Invention
This invention relates to a high density bubble domain memory and a method for making it, and in particular to an improved method for making such a memory using only two masking steps, one of which requires critical alignment.
2. Description of the Prior Art
Various systems using magnetic bubble domains are known in the art. For example, self-contained magnetic bubble domain memory chip using a decoder is shown in U.S. Pat. No. 3,701,125. Additionally, a major/minor loop memory configuration is shown in U.S. Pat. No. 3,618,054. In these memory systems, components are required which provide the functions of read, write, bubble domain propagation, transfer between storage elements, and annihilation. That is, bubble domains are generated for representation of information, and these bubble domains are generally propagated in the memory. After propagation, they are read and then annihilated or returned to their storage locations. Furthermore, these memories often require transfer functions where bubble domains are transferred from one propagation path to another, usually by the use of current carrying loops that produce magnetic field gradients for implementing the transfer.
Many components are known in the art for generating magnetic bubble domains and for detecting magnetic bubble domains. For example, a magnetoresistive sensing technique is shown in U.S. Pat. No. 3,691,540. For the function of storage, bubble domains are generally propagated using any of many well known structures. In particular, ion implanted propagation elements having generally curved paths are useful for bubble domain storage, since the line widths of these elements are generally about four bubble diameters, thereby leading to relaxed lithography requirements. Such ion implanted structures are described by R. Wolfe et al in the AIP Conference Proceedings, No. 10, Part 1, p. 339 (1973). These proceedings contain the text of the papers delivered at the 18th Annual Conference on Magnetism and Magnetic Materials, held in Denver, Colo., in 1972. Furthermore, U.S. Pat. No. 3,828,329 describes propagation structures using ion implanted regions.
The processes used for making magnetic bubble domain chips have developed through the years so that single level masking techniques are now described for making bubble domain memories in which the propagation elements are separated from one another (gapped propagation elements). In such techniques, magnetic sensors are deposited using the same mask that is used for depositing the magnetic propagation elements. Also, since the propagation elements are not in contact with one another, conductors can be placed directly over the propagation elements without shorting any electrical currents. This means that the bubble domain chip can be fabricated using only a single critical masking step.
However, with the exception of aforementioned Ser. No. 537,797, (U.S. Pat. No. 3,967,002) the prior art does not address the problem of making high density magnetic bubble domain chips where the propagation elements are contiguous to one another. In such systems it is difficult to place conductors directly on the propagation elements, since electrical shorting may occur. Additionally, several critical masking steps are usually required in order to define the sensors, propagation elements, and conductors used for bubble domain transfer and sensor current. Furthermore, it is usually necessary to provide a "protect" mask to protect the magnetoresistive sensor when the electrical conductors are formed. Because these are critical problems when bubble domain technology is to be used to provide economical memory structures having high density, the present invention seeks to provide an improved bubble domain memory in which all necessary functions are provided, and which can be made by a process using a minimum number (two) of masking steps, only one of which requires critical alignment.
Accordingly, it is a primary object of this invention to provide an improved process for fabricating high density magnetic bubble domain chips in which only one critical masking step is required.
It is another object of this invention to provide a process for fabricating a magnetic bubble domain chip having contiguous propagation elements, requiring a minimum number of masking steps.
It is still another object of the present invention to provide a high density magnetic bubble domain chip having components for generation, reading, propagation, transfer, and annihilation, all of which components do not require a resolution less than about 4d, where d is the bubble domain diameter.
It is a further object of this invention to provide an improved process for fabricating a high density magnetic bubble domain chip using ion implanted propagation elements which are contiguous to one another, magnetic sensors and annihilators, and current carrying lines for generation, sensing, and transfer.
It is another object of this invention to provide a bubble domain memory having improved means for transfer of information from one storage register to another.